By studying coral reefs off the north-west Australian coast, marine scientists have concluded that fewer sharks leads to an increase in mid size fish which in turn reduces the number of fish which eat the algae that damage the reefs. The Australian and Canadian researchers say their findings point to a need for more marine parks.

Transcript

TONY EASTLEY: Australian research has found that a decline in shark populations can damage coral reefs. The marine scientists say their study is the first evidence of the role sharks play in the reef environment.

AM's Samantha Donovan:

SAMANTHA DONOVAN: The research was conducted on coral reefs about 300 kilometres off the north-west Australian coast.

Dr Mark Meekan from the Australian Institute of Marine Science led a team of Canadian and local researchers. He says the reefs have been visited for centuries by Indonesian shark fishermen.

MARK MEEKAN: They are still allowed to do it using the same traditional methods. That is these little sailing canoes, these little dug-outs, they sail down from places like Rote in West Timor every winter and try and catch sharks.

SAMANTHA DONOVAN: Dr Meekan says the shark fishing has had a dramatic effect on the population of reef sharks like the hammerhead and whaler.

MARK MEEKAN: The funny thing is that although these guys are just in sailing canoes and putting out long lines, they're very, very effective at removing sharks. and the reefs where they're allowed to go, we've basically found that they're wiped out pretty much most of the sharks that used to be there. And that's had some very interesting impacts on the rest of the fish communities.

SAMANTHA DONOVAN: What's happened to them?

MARK MEEKAN: We've found that the smaller predatory fish, the things like the emperors and the snappers, are now way more abundant on those reefs without sharks than they are on reefs just nearby where no fishing is going on.

Now you might think that's a really good thing, you know, everyone likes to have more snappers and more emperors because they're the sorts of fish we like to eat. But unfortunately, the effects haven't stopped there. They've flowed right down the food chain and now they're affecting things like the algae eating fish that are further on down the food chain on the reef.

SAMANTHA DONOVAN: So the populations of some of the bigger fish have actually increased whereas the smaller fish, the population's gone down; is that right?

MARK MEEKAN: That's exactly right. Now that's a worry because these reefs are not just subject to fishing, they go through all sorts of disturbances like cyclones, bleaching and that sort of thing. And what happens there is that these natural disturbances kill off live coral. Algae grows on the reefs as the coral dies and the herbivores, these algae eating fish, are very important because what happens is they clear a bit of space in that algae and allow the coral areas to regrow.

Now if there aren't enough algae eating fish out there, that could be a real problem for the recovery of reef from these types of disturbances.

SAMANTHA DONOVAN: So does that mean sharks should be protected in order to protect coral reefs?

MARK MEEKAN: Well, I think it does show that, you know, it's really critical to protect sharks.

Look, coral reefs face a future, a very uncertain future but there is something we can do about protecting reef sharks.

A lot of these animals are very resident on reefs, they don't go very far. So even by creating small marine parks for these, to protect these animals we are in actual fact increasing the resilience of these reefs and we're creating some hope for the future.

SAMANTHA DONOVAN: The research is being published in the journal PLOS ONE.